Field of the Invention
The present invention relates to a solid state drawing process for polymer billets and oriented polymer compositions resulting from the drawing process.
Description of Related Art
Cross sectional dimensions of a polymer composition necessarily reduce upon drawing the polymer composition. That means that the final drawn article will have smaller cross sectional dimensions than the polymer composition prior to drawing. In drawing films or small dimension articles such a reduction in cross sectional dimensions is not particularly problematic. However, when trying to draw a polymer composition into a final drawn article having large cross sectional dimensions the process can become cumbersome because it requires billets having very large dimensions prior to drawing. The problem is particularly evident when trying to combine into a continuous process both extrusion of an initial polymer billet and drawing the polymer billet. Consider, for example, a solid state drawing process for preparing articles in the form of oriented polymer compositions (OPCs).
A solid state drawing process typically requires extruding a polymer billet, conditioning that polymer billet to a drawing temperature and then drawing the polymer billet to obtain an OPC. Generally, a billet is at a higher temperature upon extrusion than the target drawing temperature for the billet, so thermally conditioning the billet is necessary. Conditioning the billet to a uniform drawing temperature is necessary to achieve uniform orientation throughout the resulting OPC after drawing. However, rapidly cooling an extruded billet to a uniform temperature becomes increasingly challenging as the cross section of the billet increases. The core temperature of a billet is dependent on the difference in temperature between the core and the surface, the distance from the surface to the core, the heat transfer coefficient of the billet material and the heat transfer coefficient at the surface where heat is being removed from the billet. The greater the distance from the surface to the core the more time is necessary to reach a uniform temperature across a billet cross section (equilibrium temperature). For example, the cooling rate of a solid having an infinite width (that is, no edge effects considered) is proportional to the square of the distance form the surface to the core (see, for example, J. L. Throne, THERMOFORMING, Chapt. 3, pp 65, Hanser Publisher, Numich, 1987). Nonetheless, rapid cooling of a billet to a uniform cross sectional temperature equal to a drawing temperature is necessary if extrusion and drawing are to occur together in a continuous process at a reasonable production rate and with a reasonable equipment footprint.
Methods of actively cooling a billet upon extrusion are known. For instance, an early reference describing an OPC drawing process describes exposing a polymer billet to a chilled water bath immediately upon drawing (see, Ward, et al., SOLID PHASE PROCESSING OF POLYMERS, Hanser publishing, Munich (2000), Chapter 9 page 359). Other references merely describe “adjusting the temperature” of a billet to a drawing temperature, a process that typically requires heating a billet to a drawing temperature (see, for example, United States patent application 2005/0192382). Modifying the temperature of a billet's surface can occur relatively rapidly, but creates a thermal gradient between the billet's surface and the billet's core. As a result, adjusting the temperature (or “conditioning”) a billet to a drawing temperature often requires super cooling the billet surface until the core reaches a temperature close to the drawing temperature and then reheating the billet exterior to bring the surface portion back up to the drawing temperature. Cooling the core of a billet takes more time as the billet cross section increases. Therefore, conditioning a billet to a drawing temperature can be time consuming for large cross section billets.
In a continuous process, an extruded billet must undergo temperature conditioning to its drawing temperature as it travels from an extruder to a drawing device. As the time for temperature conditioning increases with large cross section billets, the distance between the extruder and drawing device must also increase or the production rate must slow down. Therefore, continuous processes for preparing and drawing large cross section billets can require extremely long process lines or extremely low production rates. It is desirable to more efficiently condition an extruded billet to a drawing temperature in a manner that can be part of a continuous process of preparing a billet and then drawing the billet to prepare an OPC.